The known processes for the direct radical hydroxylation of phenol by means of the FENTON reagent (hydrogen peroxide+ferrous ions) result essentially in mixtures of pyrocatechol and hydroquinone, i.e., the hydroxylation preferentially occurs in the ortho and para positions of the phenol function. Only very small quantities of resorcinol are formed (hydroxylation in the meta position). Similarly, the photolysis of aqueous hydrogen peroxide solutions in the presence of para-cresol at pH=3-8 mainly yields 4-methyl-pyrocatechol (3,4-dihydroxy-1-methyl-benzene). Processes of this kind are described, for example, in the work "Methoden der Organischen Chemie" (HOUBEN-WEYL), fourth edition, 6/1c, pages 30 to 34.
The ionic hydroxylation of phenols using hydrogen peroxide and carboxylic acids in the presence of a strong inorganic acid has also been studied. As an intermediate product, an organic peracid is formed, which is a very powerful oxidizing agent and has a tendency to degrade the diphenols formed. This process also essentially yields derivatives dihydroxylated in the ortho and para positions and yields virtually no meta derivative.
Other processes for the ionic hydroxylation of phenol use strong inorganic acids as catalyst, resulting in the formation of peroxonium ions. These processes make it possible to modify the pyrocatechol/hydroquinone ratio to some extent, but do not lead to the formation of resorcinol (cf. for example, the article "Hydroquinone et Pyrocatechine par Hydroxylation Directe du Phenol", Chimie-Actualites, Nov. 3, 1976, pages 47-49).
In anhydrous hydrofluoric acid as solvent, phenol is also hydroxylated by hydrogen peroxide into a mixture of hydroquinone and pyrocatechol (J. Org. Chem., 35, 4028, 1970).
Similarly, the ionic hydroxylation of paracresol gives 20% of methylhydroquinone and 41% of 4-methyl-pyrocatechol (HOUBEN-WEYL, loc. cit. page 34).
Tests on the hydroxylation of aromatic hydrocarbons with hydrogen peroxide in a superacidic medium have been reported by G. A. Olah and his collaborators ("Oxyfunctionalization of Hydrocarbons-8-Electrophilic Hydroxylation of Benzene, Alkylbenzenes and Halobenzenes with H.sub.2 O.sub.2 in Superacids", J. Org. Chem., 43, 865, 1978). Thus, toluene results in a 67% yield of a mixture of cresols, with an o/m/p ratio of 71/6/23.